56 ORDERS OF INSECTS. 



are arranged in a radiating form, so that they spread 

 and fold up in the manner of a fan. The metamor- 

 phosis of the Ortlioptera is incomplete. 



In a third order_, that of the Neuroptera, both 

 pairs of wings are usually of the same size and of a 

 similar membranous texture_, so that all these organs 

 are applicable to the purposes of flight ; they are all 

 traversed by several nervures, which are usually united 

 by an immense number of smaller ones, so that the 

 wings present the appearance of a network. The 

 metamorphosis in these insects is sometimes complete 

 and sometimes incomplete. 



In the fourth order, the Hymenoptera, the wings 

 are also membranous, but the hinder ones are always 

 considerably smaller than the anterior ; their nervures 

 are comparatively few in number, and the network 

 which they describe upon the surface of the wing con- 

 sists of but few meshes (called cells), the form of 

 which is usually very determinate, and of considerable 

 importance in the subdivision of the insects of this 

 order. The metamorphosis in the Hymenoptera is 

 always complete. 



Besides these four principal orders, there are two 

 smaller ones belonging to the mandibulate section of 

 insects; one of these, that of the Strepsiptera, is 

 very closely allied to the Coleoptera, if indeed it should 

 not rather be considered as an integral portion of that 

 order; whilst the other, that of the Physopoda, 

 may stand between the Orthoptera and Neuroptera. 

 It will be unnecessary for our present purpose to give 

 the characters of these small groups in this place. 



In the insects of the second group, the organs of 

 the mouth have undergone the modifications already 

 described (see p. 14-17), to adapt them for the suction 



